1. Field of the Invention
The present invention relates to a masking tape for use in manufacturing a printed board having an electrically conductive layer formed on its surface and having formed, at specified points therein, through holes through which upper and lower circuitry patterns are to be electrically connected with each other, and more particularly to a masking tape for use in a process of filling up such through holes with a filler material, for example, an etching resist, and the like.
2. Description of the Prior Art
In order to obtain a printed board, an electrically insulating substrate such as phenolic resin, epoxy resin, or the like, coated with copper foil on its upper and lower side surfaces, is formed with holes that pass through the substrate at predetermined points thereof. The the substrate is then subjected to a photoetching treatment so as to effect a required connection between upper and lower circuitry patterns through the holes. Thus, the aimed printed board is obtained.
In the manufacturing process of forming such printed boards, a process of filling up the through holes formed therein with etching resist ink is one of factors that determine the final quality of products. Heretofore, the filling process has been conducted by, for example, methods illustrated in FIGS. 2 to 4.
According to the method of FIG. 2, masking tapes are applied to such portions as positioning holes for a pattern printing, end faces of a printed board, and the like which it would be troublesome to fill up with the ink. Then, one roller 4' of a pair of rollers 4, 4' for rotary drive is partially soaked in etching resist ink 3 stored in a tank 5, and thereafter a substrate 1 formed with through holes 2 is passed between the rollers 4, 4'. After completion of filling up the through holes 2 with the ink 3, that ink 3 which is sticking to the surface of the substrate 1 is wiped out by passing the substrate between a pair of squeezing blades 6, 6'. Then, the ink 3 thus having filled up the through holes 2 is surplus cured, and cured ink 3' is abraded off by means of an abrader, buff, brush, or the like and the substrate finally washed with water.
The method of FIG. 3, in which like parts are denoted by the same reference numerals, renders unnecessary such work as taping the positioning holes, the end faces of the substrate, and the like. According to this method, a substrate 1 coated with copper foil on its side surfaces and having through holes 2 formed at fixed points is mounted on a base 8 with a required space 7. A mask 10 formed with holes 9 is superposed on the substrate 1 such that the holes 9 coincide with the through holes 2, so that there is effected a covering over positioning holes 11 formed on peripheral edges of the substrate 1 for subsequent processes and on end faces 12. A squeezer 13 is passed over the mask 10, for application of pressure onto the surface of the mask 10 thus superposed, so that the through holes 2 are filled with ink 3 by way of the holes 9. Then, the mask 10 is taken off, and after curing the ink 3, excess cured ink is removed by abrading and washing as in the foregoing method.
Furthermore, according to the method of FIG. 4, in which like parts are also denoted by the same reference numerals, steps are taken for covering at least positioning holes 11 on the peripheral edges and end faces 12 of the substrate 1 with a mask plate 13; arranging a first squeezer 14 and a second squeezer 14' across the substrate 1; and filling up the through holes 2 with the ink 3 by squeezing the substrate 1 with the first squeezer 14 for application of pressure to the substrate 1 while constantly keeping the second squeezer 14' behind the first squeezer 14 in the proceeding direction so that the second squeezer 14' can support the substrate 1 and at the same time scape off excess ink passed from the through holes 2.
However, the following problems are encountered in conducting the process of filling up the through holes by the use of a commercially available adhesive tape according to the method illustrated in FIG. 2:
(1) While it is possible to mechanize the taping process, it is extremely difficult to mechanize the process of peeling off the tape that adheres to a substrate after curing of the ink filled within the through holes. Because the adhesive tape would be in a state of being welded due to exposure to high temperature during the process of ink curing, even manual peeling of such tape is hard to effect and mechanical peeling all the more so. Thus the difficulty in mechanization of the peeling process is an obstacle to automatization of the printed board manufacturing process that includes the process of filling up the through holes.
(2) When removing excessive ink cured and sticking to the surface of the substrate by abrading and washing after curing the ink filled within the through holes, and at the same time scraping off the adhesive tape applied, circumstances often occur in which the tape remains stuck if it is composed of strong tape material and has an adhesive agent of high adhesion. Meanwhile, if the tape is composed of a strong tape material and an adhesive agent of low adhesion, the surface of the substrate can be clean but pieces of the tape material taken off remain inside the abrader, buff, and the like. On the other hand, if the tape material is weak while the adhesive agent is of high adhesion, the adhesive agent remains stuck to the surface of the substrate, thereby preventing the surface from being clean. If both the tape material and the adhesive agent are weak, the surface of the substrate can be clean but pieces of the tape material taken off remains inside the abrader, buff, and the like, thus causing problems.
In the case of the method illustrated in FIG. 3, no problem arises due to taping as mentioned above, nor do problems such as entering of the ink 3 into the positioning hole 11 or sticking thereof to the end faces 12 of the substrate when the through holes 2 are filled up with the ink 3. However, when the ink 3 filled in the through holes 2 is cured as partially split on or spreading over the surface of the substrate, it is necessary to repeat the scraping process in order to remove the spreading ink 3 from the surface of the substrate 1. Apparently, such repetitious scraping renders the copper layer on the surface thinner. Accordingly, measures must be taken to avoid this problem. If the ink 3 spilled or spread out of the through holes 2 is cleaned with the squeezing blades 6, 6' (per FIG. 2) or the squeezers 14, 14' (per FIG. 4) prior to a hardening process to this end, the scraped ink may enter in the positioning holes 11 or stick to the end faces 12 of the substrate, eventually making the mask useless. Hence, it has been necessary to clean the positioning holes 11 manually, taking much time and labor, with the result that such a manufacturing process is not feasible.
When it comes to the method depicted in FIG. 4, although the problem does not exist that the ink 3 enters into the positioning holes 11, 11' or sticks to the end faces 12, 12' from the upper side of the substrate, it may easily happen that the ink 3" scraped off by the second squeezer 14' gets into the positioning hole 11' or sticks to the end face 12' from the lower side of the substrate 1, since the positioning hole 11' and the end face 12' located in the slidingly moving direction of the squeezer 14'. If the ink 3 acutally gets into the positioning hole 11', then the aforementioned problem arises. Further, if the ink 3" sticking to the end face 12' is hardened, the hardened ink may be partially removed during the printing process so as to give rise to scumming and pinholes, thus adversely affecting the printing accuracy that depends on abutting positioning. As a result, it becomes necessary to clean the same manually in the same manner as in the case of the positioning holes, likewise taking much time and labor.